The present invention generally relates to processes for the production of a high-strength alloy that may be used as a gas purification membrane, as an electrode for numerous applications including the generation of heat energy or other electrochemical processes, and more particularly to the preparation and use of two-phase palladium-boron alloys which have greater strength and hardness than other palladium metals or alloys and which thus can be advantageously utilized in a variety of applications including hydrogen purification membranes or electrodes.
With increased use of electrical processes and hardware, processes utilizing the excellent reliability and conductivity of palladium, an extremely valuable but expensive metal, have become of increasing importance, particularly when used as an electrode. However, it has long been known that the hardness of palladium is often less than optimal for many of these processes. Accordingly, there has been a distinct need in this field to develop palladium alloy electrodes that are harder and more resilient than pure palladium while still offering the superior electrical characteristics of pure palladium.
In addition, interest has increased in the quick and efficient production of hydrogen, which has, because of its many industrial and scientific applications, assumed greater importance. Hydrogen is typically purified from surrounding gas by using a membrane permeable to hydrogen, but not to the other gases. In this process, the hydrogen passes through the membrane and is collected on the other side. With respect to hydrogen production, there is much interest in methods of increasing the hardness and durability of these membranes which are again typically composed of palladium. One proposed solution to overcoming the hardness problem would be to harden the palladium metal without affecting its hydrogen purification characteristics, which would allow for thinner membranes than those of pure palladium. This would allow either the same amount of hydrogen to be purified at a great cost savings, or a larger amount of hydrogen could be purified for the same cost. However, suitable methods for developing palladium or palladium alloys with sufficient hardness have not yet been achieved.
Further, the demand for energy increases each year while the world""s natural energy sources such as fossil fuels are finite and are being used up. Accordingly, the development of alternative energy sources is very important and a number of potential new energy sources are under study. Although there have been many attempts to develop a palladium compound which can be utilized in processes to generate heat, such as through the introduction of aqueous deuterium, none of these attempts have been successful or repeatable, and there is thus a distinct need to develop palladium alloys which can be utilized for the generation of heat as a potential energy source.
Previously, it has been known to prepare single-phase alloys made of palladium and other minor elements. For example, the prior art includes various palladium alloys which include boron, such as Weber et al. U.S. Pat. No. 5,518,556 (a boron-containing surface layer), Hough et al. U.S. Pat. No. 4,341,846 (an electroless boron/palladium plating material), Smith Jr. et al.. U.S. Pat. No. 4,396,577 (a brazing alloy containing boron, palladium and other metals) and Prosen U.S. Pat. No. 4,046,561 (an alloy for porcelain applications containing boron, palladium and other metals).
However, what is lacking in the prior art is a pure boron/palladium composition of sufficient strength to be used as a reactive structure rather than a coating material, and which may be used in thin hydrogen purification membranes or as an electrode in a heat-generating process. There thus remains a distinct need to develop palladium alloys which can be utilized advantageously in a variety of applications where pure palladium is unsuitable either because of the expense or insufficient hardness.
In accordance with the invention, there is provided a two-phase alloy comprised of palladium and boron wherein the boron is in solid solution in the palladium and wherein each phase of the two-phase structure has the same crystal structure as the other phase but has a different set of lattice parameters from the other phase. In addition, a method of preparing the two-phase alloy of the invention is also provided wherein the boron in powder form is preferably placed in an airless compartment, palladium in sponge form is placed in the compartment overlying the boron, the boron and palladium are melted together to form a mixture via a heating apparatus such as an electric arc, the mixture is cooled to solidification, turned over for complete mixing, and the melting, cooling and turning process is preferably repeated until a mixture with the desired homogeneity is attained. In the preferred process, the amount of boron is such that it is insufficient to form a compound of boron in the palladium, but sufficient to remain in solid solution with the palladium.
In the particularly preferred embodiment, the composition of the present invention comprises 0.1 to 0.8 by weight percent boron, and 99.2 to 99.9 percent by weight percent palladium, and the palladium and boron comprise at least 99.9% of the composition. It is also preferred that the second phase forms crystallites which are on average at least twice as large as the crystallites of the first phase, and that the diameter of the crystallites in the first phase is in the range of 10 to 100 Angstroms.
In a particularly preferred method or preparation in accordance with the invention, the palladium and boron are placed on a copper hearth in a mixing chamber which is part of an arc melting means. The arc melting is then performed between about 2079xc2x0 C. and 2200xc2x0 C., for a period of between about 4 and 10 minutes. The melting, cooling and turning steps are preferably repeated roughly 3-10 times. After a complete mixture results from melting, turning, and cooling, the composition may also be swaged to reduce the diameter of the alloy. The alloy is annealed at elevated temperature to reduce the residual stress, and then undergoes a final cooling to room temperature. The annealing is performed between about 650 and 700xc2x0 C., and for less than about three hours.
Preferably the alloy composition of the present invention can be formed into a membrane for use in the purification of hydrogen, or can be made into an electrode useful for numerous purposes, including the loading of the electrode with deuterium for the generation of heat energy, or other standard electrochemical purposes.
Further features and advantages of the present invention will be set forth in, or apparent from, the detailed description of preferred embodiments which follows below.